Reports of roof collapses in the aftermath of multi-foot snowfall points to one of the dangers posed by major snowstorms.

Take a 20- to 30-inch snowfall, which can easily hold 2 to 3 inches of water equivalent. For each square foot of roof surface, a weight of roughly 10 to 15 pounds must be borne by a building's roof and other structural members.

With surface areas of schools and commercial buildings reaching many thousands of square feet, tons of snow burden quickly piles up. Under-engineered roofs can give way under the load.

Strong winds and drifting can heighten the danger all the more. Wind-drifted snow can become highly concentrated over irregular roofs with loads locally much greater than the average load.

Story by AccuWeather.com Senior Meteorologist Jim Andrews.